The invention relates to light-emitting diode (“LED”) devices, and more particularly to LED devices that mix light from a number of LEDs to produce white light.
Light is fundamental to imaging, and the nature of light affects the resultant image quality. For example, the color of a light source used in imaging can affect the hue of the image. The color of a light source is often characterized by its color temperature and its color rendering index (“CRI”). Color temperature is an indication of the light provided by the light source compared to light that would be emitted by a black body radiator at a certain temperature. For example, a 100-Watt incandescent bulb has a color temperature of about 2870 degrees Kelvin, which means the light emitted by the incandescent bulb is about the same color (generally yellowish-white) as a black body heated to 2870 degrees Kelvin.
Color temperature is useful for determining the correct type of film to use in photography, and for specifying the right light source types for imaging applications. For example, daylight film is color balanced (i.e. will accurately reproduce the color of a subject in an image) when used with a light source having a color temperature of 5,500 degrees Kelvin, such as daylight or light from a strobe light. Tungsten film is color balanced when used with a light source having a color temperature of about 3,200 degrees Kelvin, which is an orange light often described as “warm.” Stage lighting, flood lighting, and home lighting are typically warm light sources. If such light sources are used to illuminate a subject imaged with daylight film, the image will often have an orange cast. Similarly, if tungsten film is used to image a subject illuminated by strobe lights or daylight, the image will appear bluish.
However, light sources of the same color temperature can vary widely in the quality of light emitted. One may have a continuous spectrum, while the other just emits light in a few narrow bands of the spectrum, yet both have the same color temperature. A useful way to determine the quality of a light source is its CRI. To determine a CRI value, observers view 8 standard pastel colors under the light source being rated and under light from a blackbody source (such as an incandescent lamp) having the same color temperature. The CRI is calculated, roughly speaking, by averaging the observers' estimation of the extent of the differences in the appearance of the colors under the two lights. The CRI can only be used to compare two light sources that have the same color temperature. It serves as a quality distinction between light sources emitting light of the same color. The highest CRI attainable is 100.
Electronic flash tubes have been used by photographers since the 1950s to illuminate a subject when making an image on film or with a digital imaging device. A typically electronic flash tube has a gas, such as xenon, or combination of gasses, inside a tube with electrodes on both ends and a metal trigger plate in the middle of the tube. A voltage is applied to the electrodes to ionize the gas in the flash tube. Electrons flow through the ionized gas and excite the gas ions, which emit visible light. The emitted light is typically in a very narrow band of wavelengths corresponding to the atomic transition levels in the gas. Hence, the light emitted from a flash tube is essentially fixed.
An LED is a semiconductor device capable of emitting light when an electric current flows through it. LEDs are used in many applications, such as electronic displays, traffic signals, and video signs. LEDs emit monochromatic light. One way to characterize the light output of an essentially monochromatic source is by the full-width half maximum (“FWHM”) value of the emitted light. The FWHM is the width of the spectrum taken halfway from the maximum (peak) emission. The FWHM of light emitted by an LED falls within a narrow range, typically about 20-50 nanometers (“nm”). In some applications, a phosphor is applied to the LED to broaden the FWHM and shift the peak wavelength of the emitted light.
LED flash modules, such as are used in or with cameras, digital cameras, and camera-containing devices such as mobile telephones and personal digital assistants, use phosphor-converted LEDs to create a “white” flash. The LED flash module typically has one or more blue LEDs (primary emitters) with phosphors (secondary emitters) deposited on the blue LEDs. The spectral content of the flash light is determined by the secondary emissions of the phosphors and primary emissions of the LEDs. Using these techniques, flash modules obtain color temperatures in the range of about 3500 degrees Kelvin to about 8500 degrees Kelvin. However, the CRI is usually only about 65. An LED flash module providing white light with a higher CRI is desirable.